You CANNOT cheap out on an electric fan. Been there, done that. Unless you can find and retrofit some OEM fan setup from a Junkyard (Ford Taurus/Windstar and some kind of Volvo if I remember correctly) on the cheap, you will be in hundreds of dollars which as Whistler said, is quite a bit of fuel.
For reference, here is a screenshot from my Fuelly log. The bottom row shows how much you could save per 1MPG gain and the amount of money you would have saved. Keep in mind the MPG I logged is uncorrected for my taller tires, but, still even saving 3mpg, will gain you $5 per fill. 2 fills a month x 12 months = only $120 a year.